Tire condition monitoring apparatus

ABSTRACT

A transmitter, which is operated by a battery, wirelessly transmits data including data that represents the temperature T in a tire and data that represents the voltage V of the battery. A receiver judges whether or not the life of the battery is ending depending on the battery voltage V read from the received data. More specifically, the receiver performs the judgment regarding the life of the battery based on a comparison between the battery voltage V and a variable voltage reference value V0, which is selected in accordance with the temperature T in the tire. The judgment regarding the life of the battery is thus accurately performed.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to apparatuses for monitoring thecondition of a tire attached to a vehicle, and, more particularly, totechniques for judging the condition of a battery installed in atransmitter that is attached to a tire.

[0002] Conventionally, a wireless, tire condition monitoring apparatusinforms a driver of the condition of each tire attached to a vehiclethrough an indicator located in a passenger compartment. A transmitteris attached to each tire. The transmitter measures the pressure andtemperature in the associated tire and wirelessly transmits datarepresenting the measurements. A receiver is located in the vehicle'sbody frame and receives the data from the transmitter. The receiverobtains necessary information from the data and instructs the indicatorto indicate the information as needed.

[0003] Since each transmitter is powered by a battery, the transmitterstops operating if the associated battery's life ends. Thus, when thebattery's life is ending, the driver needs to be informed of thecondition of the battery through the indicator, which is located in thepassenger compartment. Accordingly, the conventional monitoringapparatus transmits data representing the voltage of each battery,together with the data representing the condition of each tire. If thevoltage of the battery is equal to or smaller than a predeterminedreference value, the receiver judges that the life of the battery isending. The receiver then instructs, for example, the indicator toindicate the judgment.

[0004] However, the battery voltage is affected by the ambienttemperature to which the battery is exposed. That is, the lower theambient temperature is, the lower the battery voltage is. Particularly,the temperature variation in the tire in which the battery is located isrelatively large. Thus, the condition of the battery cannot beaccurately judged if the judgment is based on only the comparisonbetween the battery voltage and the reference value. In other words, ifthe judgment is based only on such a comparison, the condition of thebattery may be erroneously judged.

BRIEF SUMMARY OF THE INVENTION

[0005] Accordingly, it is an objective of the present invention toprovide a tire condition monitoring apparatus that accurately judges thecondition of a transmitter battery.

[0006] To achieve the foregoing and other objectives and in accordancewith the purpose of the present invention, the invention provides a tirecondition monitoring apparatus for monitoring a condition of a tireattached to a vehicle. The apparatus includes a transmitter and acontroller. The transmitter is operated by a battery and detects atleast the temperature in the tire and a voltage-related value thatvaries in accordance with voltage of the battery. The controller judgeswhether or not the life of the battery is ending depending on thevoltage-related value. The judgment regarding the life of the battery isperformed in accordance with the temperature in the tire.

[0007] Other aspects and advantages of the invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention, together with objects and advantages thereof, maybest be understood by reference to the following description of thepresently preferred embodiments together with the accompanying drawingsin which:

[0009]FIG. 1 is a view schematically showing a first embodiment of atire condition monitoring apparatus according to the present invention;

[0010]FIG. 2 is a block diagram showing a transmitter of the apparatusof FIG. 1;

[0011]FIG. 3 is a block diagram showing a receiver of the apparatus ofFIG. 1;

[0012]FIG. 4 is a graph representing the relationship between batteryvoltage and ambient temperature;

[0013]FIG. 5 is a graph representing the relationship between batteryvoltage and battery discharge level;

[0014]FIG. 6 is a flowchart showing a battery life judging process ofthe first embodiment;

[0015]FIG. 7 is a flowchart showing a battery life judging process of asecond embodiment according to the present invention; and

[0016]FIG. 8 is a flowchart showing a battery life judging process of athird embodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] A first embodiment of the present invention will now be describedwith reference to FIGS. 1 to 6. As shown in FIG. 1, a tire conditionmonitoring apparatus includes four transmitters 3 and a receiver 4. Eachtransmitter 3 corresponds to a tire 2 of a vehicle 1, and the receiver 4is installed in the body frame of the vehicle 1. More specifically, eachtransmitter 3 is secured to a wheel to which the corresponding tire 2 isattached such that the transmitter 3 is located in the tire 2. Thetransmitter 3 measures the pressure and temperature in the tire 2 andwirelessly transmits data representing the measurements to the receiver4.

[0018] As shown in FIG. 2, each transmitter 3 includes a controller 3,which is, for example, a microcomputer. The controller includes, forexample, a central processing unit (CPU), a random access memory (RAM),and read only memory (ROM). The controller 10 stores specificidentification codes and identifies the four transmitters 3 that areinstalled in the vehicle 1 in accordance with the associatedidentification codes.

[0019] A pressure sensor 11 measures the pressure in the tire 2 andoutputs pressure data that represents the measurement to the controller1. A temperature sensor 14 measures the temperature in the tire 2 andoutputs temperature data that represents the measurement to thecontroller 1.

[0020] A battery 15 supplies power to each transmitter 3. A voltagesensor 16 measures voltage of the battery 15 and outputs voltage datathat represents the measurement to the controller 10.

[0021] The controller 10 transmits data that includes the pressure data,the temperature data, the voltage data, and an identification code to atransmitting circuit 12. The transmitting circuit 12 encodes andmodulates the data received from the controller 10. The data is thenwirelessly transmitted through a transmitting antenna 13.

[0022] The controller 10 instructs the pressure sensor 11, thetemperature sensor 14, and the voltage sensor 16 to perform measurementsevery predetermined time interval (for example, every fifteen seconds).Further, the controller 10 instructs the transmitting circuit 12 toperform a periodic transmission every time the pressure sensor 11completes a predetermined number of measurement cycles (for example,forty cycles). When detecting an abnormal pressure or temperature of theassociated tire 2, the controller 10 instructs the transmitting circuit12 to transmit data, regardless of whether or not the pressure sensor 11has completed the predetermined number of measurement cycles.

[0023] As shown in FIG. 3, the receiver 4 is powered by a battery (notshown) installed in the vehicle 1 when a key switch (not shown) of thevehicle 1, for example, is turned on. The receiver 4 includes acontroller 20, which is, for example, a microcomputer. The controller 20functions as a battery life judging means and includes, for example, aCPU, an RAM, and an ROM.

[0024] An RF circuit 21 receives data from each transmitter 3 through atleast one transmitting antenna 22. The RF circuit 21 demodulates anddecodes the data and then sends the data to the controller 20. Thecontroller 20 then reads the pressure and temperature in the associatedtire 2 from the data and judges whether or not the life of theassociated battery 15 is ending.

[0025] The controller 20 instructs an indictor 23 to indicateinformation regarding the temperature and pressure in each tire 2 andinformation regarding the life of the associated battery 15. Theindicator 23 is located to be viewed easily by the driver of the vehicle1. Further, when detecting an abnormal temperature or pressure in onetire 2, the controller 20 instructs an alarm device 20 to inform thedriver of the abnormality. The alarm device 20 is, for example, an audiodevice that produces an alarm sound or an illuminating device that emitsan alarm light. Alternatively, the indicator 23 may indicate theabnormal pressure or temperature of the tire 2.

[0026] Next, a battery life judging process performed by the controller20 will be described with reference to FIG. 6. The flowchart of FIG. 6indicates a routine of the battery life judging process. The routine isrepeated, for example, every time the receiver 4 receives data from onetransmitter 3. Further, the routine is performed independently for eachtransmitter 3 of the vehicle 1. The data transmitted from thetransmitter 3 includes its specific identification code. Thus, when thecontroller 20 receives data from one transmitter 3, the controlleridentifies the transmitter 3 in accordance with the identification code.

[0027] Subsequently, in step S1, the controller 20 reads the temperatureT in the tire 2 associated with the transmitter 3, from which thecontroller 20 has received the data, and the voltage V of the associatedbattery 15 from the received data.

[0028] In step S2, the controller 20 computes a voltage reference valueV0 in relation to the temperature T using the following equation (1).Whether or not the life of the battery 15 is ending is judged withreference to the voltage reference value V0. The numerals a, b in theequation (1) are predetermined constants that are selected depending onwhich type the battery 15 is.

V0=aT+b  Equation (1)

[0029]FIG. 4 is a graph showing the relationship between the ambienttemperature and the voltage of the battery 15. In the graph, the curveVn represents voltage variation of the battery 15 in a fully chargedstate. The curve Vu represents voltage variation of the battery 15 in an80% discharged state. FIG. 5 is a graph showing the relationship betweenthe discharge level of the battery 15 and the voltage of the battery 15.In the graph, the curve Vh represents voltage variation of the battery15 when the ambient temperature is twenty degrees Celsius. The curve Vlrepresents voltage variation of the battery 15 when the ambienttemperature is −40 degrees Celsius. The graphs of FIGS. 4 and 5 wereobtained based on test results.

[0030] As is clear from the graphs of FIGS. 4 and 5, as the dischargelevel of the battery 15 increases, the voltage of the battery 15decreases. Further, as the ambient temperature to which the battery 15is exposed decreases, the voltage of the battery 15 decreases. Theconstants a, b of the equation (1) are selected in accordance withcharacteristics of the battery 15, which are indicated by the graphs ofFIGS. 4 and 5. For example, the constants a, b are selected such thatthe voltage reference value V0 corresponds to the broken straight lineof FIG. 4. The broken line is an approximation of the curve Vu, whichrepresents the voltage variation of the battery 15 in the 80% dischargedstate. In other words, the linear function represented by the equation(1), by which the voltage reference value V0 is determined, is anapproximation of the curve Vu. In accordance with the graph of FIG. 4,the voltage reference value V0 is selected to be 2.55 volts when theambient temperature to which the battery 15 is exposed, or thetemperature T in the tire 2, is 50 degrees Celsius.

[0031] After completing step S2, the controller 20 proceeds to step S3,as shown in FIG. 6. More specifically, the controller 20 judges whetheror not the voltage V is equal to or greater than the voltage referencevalue V0. If the voltage V is equal to or greater than the voltagereference value V0, the controller 20 determines that the life of thebattery 15 is not ending yet. The controller 20 then proceeds to stepS4. In step S4, the controller 20 resets the counter number X to zero,thus discontinuing the process.

[0032] In contrast, if the voltage V is smaller than the voltagereference value V0, the controller 20 performs step S5. In step S5, thecontroller 20 adds one to the current counter number X, thus obtaining anew counter number X.

[0033] Next, in step S6, the controller 20 judges whether or not thecounter number X is equal to or greater than a predetermined number N.The predetermined number N is selected to be an integer that is notsmaller than one, which is, for example, three. If the counter number Xis smaller than the predetermined number N, the controller 20discontinues the process. However, if the counter number X is equal toor greater than the predetermined number N, it is indicated that thevoltage V of the battery 15 has been repeatedly judged to be smallerthan the voltage reference value V0 in consecutive judgment cyclescorresponding to the counter number X. In this case, the controller 20proceeds to step S7.

[0034] In step S7, the controller 20 determines that the life of thebattery 15 is ending. The controller 20 then instructs, for example, theindicator 23 or the alarm device 24 to inform the driver that the lifeof the battery 15 is ending. Afterward, the controller 20 discontinuesthe process.

[0035] As described, in the first embodiment, whether or not the life ofthe battery 15 is ending is judged based on comparison between thevoltage V of the battery 15 and the voltage reference value V0. Thevoltage reference value V0 is varied in accordance with the temperatureT in the tire 2 in which the battery 15 is located. Thus, it isaccurately judged whether or not the life of the battery 15 is ending,regardless of the ambient temperature to which the battery 15 isexposed.

[0036] The equation (1), which determines the voltage reference valueV0, is selected considering characteristics of the actual voltagevariation of the battery 15 with respect to the ambient temperature.Accordingly, the voltage reference value V0 constantly reflects theactual voltage variation of the battery 15 that is attached to theassociated transmitter 3.

[0037] The equation (1) reflects the voltage variation of the battery 15in the 80% discharged state with respect to the ambient temperature.Thus, regardless of the temperature T in the tire 2, the controller 20accurately determines that the life of the battery 15 is ending when thedischarge level of the battery 15 substantially reaches 80%. That is, inthe first embodiment, the judgment regarding the life of the battery 15is performed based on the discharge level of the battery 15.

[0038] Even if the discharge level of the battery 15 reaches 80%, thetransmitter 3 does not immediately stop operating. Thus, the driver ofthe vehicle 1 is notified that the life of the battery 15 is endingbefore the transmitter 3 actually stops operating.

[0039] The voltage reference value V0 is computed in relation to thetemperature T of the tire 2 using the equation (1). Since the equation(1) simply represents the linear function, the computing process of thevoltage reference value V0 is easy to perform.

[0040] If the voltage V has been repeatedly judged to be smaller thanthe voltage reference value V0 in consecutive judgment cycles, it isdetermined that the life of the battery 15 is ending. Thus, thedetermination is highly reliable, as compared to the case in which thedetermination is based on a single cycle of comparison between thevoltage V and the voltage reference value V0.

[0041] Alternatively, in step S6, the predetermined number N may beselected to be one. If this is the case, once the voltage V of thebattery 15 is judged to be smaller than the voltage reference value V0,it is determined that the life of the battery 15 is ending. However,since the voltage reference value V0 is selected in accordance with thetemperature T in the tire 2, the determination is still sufficientlyreliable.

[0042] A second embodiment according to the present invention willhereafter be described with reference to the flowchart of FIG. 7. In thesecond embodiment, the battery life judging routine of FIG. 7 isperformed instead of the routine of FIG. 6. Same or like referencenumerals are given to steps in FIG. 7 that are the same as or likecorresponding parts in FIG. 6.

[0043] When receiving data from one transmitter 3, the controller 20performs step S1. In step S1, as in the corresponding step of FIG. 6,the controller 20 reads the temperature T in the tire 2 associated withthe transmitter 3 and the voltage V of the associated battery 15, fromthe received data.

[0044] Subsequently, in step S11, the controller 20 judges whether ornot the temperature T is equal to or greater than a predeterminedreference value T0. The reference value T0 is a constant value which is,for example, approximately −30 degrees Celsius. If the temperature T isequal to or greater than the reference value T0, the controller 20proceeds to step S12 and judges whether or not the life of the battery15 is ending.

[0045] In step S12, the controller 20 judges whether or not the voltageV is equal to or greater than a predetermined voltage reference valueV1. The voltage reference value V1 is a constant value, unlike thevoltage reference value V0 of the routine of FIG. 6. If the voltage V isequal to or greater than the voltage reference value V1, the controller20 proceeds to step S4. In contrast, if the voltage V is smaller thanthe voltage reference value V1, the controller 20 performs step S5.

[0046] The procedure of steps S4 to S7 is the same as that of theroutine of FIG. 6. That is, if the voltage V is equal to or greater thanthe voltage reference value V1, it is determined that the life of thebattery 15 is not ending yet. However, if the voltage V has beenrepeatedly judged to be smaller than the voltage reference value V1 in Nconsecutive judgment cycles, it is determined that the life of thebattery 15 is ending.

[0047] If the controller 20 determines that the temperature T in thetire 2 is smaller than the reference value T0 in step S11, thecontroller 20 discontinues the process without performing the judgmentregarding the life of the battery 15.

[0048] As described, in the second embodiment, the judgment regardingthe life of the battery 15 is not performed as long as the temperature Tin the tire 2, or the ambient temperature to which the battery 15 isexposed, is smaller than the reference value T0. That is, if the voltageV of the battery 15 is excessively low due to the ambient temperature,the judgment is suspended. This prevents the life of the battery 15 frombeing erroneously judged to be ending when the temperature in the tire 2is relatively low.

[0049] The reference value T0 and the voltage reference value V1 areconstant values, unlike the routine of FIG. 6 in which the voltagereference value V0 is changed in accordance with the temperature T inthe tire 2. This simplifies the battery life judging process, and theoperation of the controller 20 is also simplified.

[0050] A third embodiment of the present invention will hereafter bedescribed with reference to the flowchart of FIG. 8. In the thirdembodiment, the battery life judging process of FIG. 8 is performed,instead of the routine in FIG. 6 or 7. Same or like reference numeralsare given to steps in FIG. 8 that are the same as or like correspondingparts in FIG. 6 or 7.

[0051] The routine of FIG. 8 is essentially the same as the routine ofFIG. 7 except for step S11, which is not performed in the routine ofFIG. 8. More specifically, in the routine of FIG. 8, when receiving datafrom one transmitter 3, the controller 20 performs step S1. That is, thecontroller 20 reads the voltage V of the battery 15 associated with thetransmitter 3 from the received data.

[0052] Subsequently, in step S12, the controller 20 judges whether ornot the voltage V is equal to or greater than the predetermined voltagereference value V1. As in the routine of FIG. 7, the voltage referencevalue V1 is a constant value. If the voltage V is equal to or greaterthan the voltage reference value V1, the controller 20 proceeds to stepS4. In contrast, if the voltage V is smaller than the voltage referencevalue V1, the controller 20 proceeds to step S5.

[0053] The procedure of steps S4 to S7 of FIG. 8 is the same as thecorresponding procedure of FIG. 6. That is, if the voltage V is equal toor greater than the voltage reference value V1, it is determined thatthe life of the battery 15 is not ending yet. However, if the voltage Vhas been repeatedly judged to be smaller than the voltage referencevalue V1 in consecutive judgment cycles corresponding to thepredetermined number N, it is determined that the life of the battery 15is ending.

[0054] As described, in the third embodiment, it is determined that thelife of the battery 15 is ending if the voltage V has been repeatedlyjudged to be smaller than the voltage reference value V1 in theconsecutive judgment cycles. Thus, as compared to the case in which thejudgment regarding the life of the battery 15 is performed based on asingle cycle of comparison between the voltage V and the voltagereference value V1, the judgment is reliable.

[0055] In the third embodiment, the judgment regarding the life of thebattery 15 is performed regardless of the temperature T in the tire 2.However, even when the atmospheric temperature is relatively low, thetemperature T gradually increases if the vehicle 1 is maintained in anoperating state. Thus, as long as the vehicle 1, or the receiver 4,remains in the operating state, the voltage V of the battery 15 startsto increase after a relatively short period of time in which the voltageV of the battery 15 is excessively decreased due to the ambienttemperature, unless the life of the battery 15 is actually ending.Accordingly, if the predetermined number N is selected to be arelatively large number, for example, eight to ten, the life of thebattery 15 is prevented from being erroneously judged, even if theambient temperature is relatively low.

[0056] The present invention is not restricted to the illustratedembodiments but may be modified as follows.

[0057] In the routine of FIG. 6, the voltage reference value V0 does notnecessarily have to be determined using the equation (1) as illustrated.That is, the equation (1) may be modified depending on the type orcharacteristics of the battery 15, which is attached to the transmitter3.

[0058] In the routine of FIG. 6, the voltage reference value V0 may bedetermined without using the equation (1). For example, a memory of thecontroller 20 stores data that indicates the curve Vu in the graph ofFIG. 4, which represents the voltage variation of the battery 15 in the80% discharged state. The controller 20 selects the voltage referencevalue V0 in accordance with the stored data.

[0059] In the routines of FIGS. 6 to 8, the predetermined number N maybe modified depending on conditions under which the transmitter 3 isused, as long as the predetermined number N is an integer.

[0060] In the routine of FIG. 7 or 8, the voltage reference value V1 maybe modified depending on the type or characteristics of the battery 15.

[0061] In the routine of FIG. 7, the reference value T0 may be modifieddepending on conditions under which the transmitter 3 is used or thetype or characteristics of the battery 15.

[0062] Each transmitter 3 does not necessarily have to include thevoltage sensor 16, as long as each transmitter 3 has a sensor thatmeasures a value that varies in relation to the voltage V of the battery15. For example, the transmitter 3 may be provided with a resistancesensor that measures resistance in an electric circuit of thetransmitter 3. In this case, the transmitter 3 transmits data thatrepresents the resistance, instead of data that represents the voltage Vof the battery 15. The receiver 4 reads the resistance value from thereceived data and compares the value with a reference value to determinewhether or not the life of the battery 15 is ending. The reference valuemay be varied in accordance with the temperature T in the tire 2 or maybe a constant value.

[0063] The battery life judging routines of FIGS. 6 to 8 may beperformed by the transmitters 3, instead of the receiver 4. In thiscase, when each transmitter 3 determines that the life of the associatedbattery 15 is ending, the transmitter 3 informs the receiver 4 of thestate of the battery 15.

[0064] Therefore, the present examples and embodiments are to beconsidered as illustrative and not restrictive and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalence of the appended claims.

What is claimed is:
 1. A tire condition monitoring apparatus formonitoring a condition of a tire attached to a vehicle, the apparatuscomprising: a transmitter, which is operated by a battery and detects atleast the temperature in the tire and a voltage-related value thatvaries in accordance with voltage of the battery; and a controller,which judges whether or not the life of the battery is ending dependingon the voltage-related value, wherein the judgment regarding the life ofthe battery is performed in accordance with the temperature in the tire.2. The apparatus as set forth in claim 1, wherein the transmitterwirelessly transmits data, which includes data that represents thetemperature in the tire and data that represents the voltage-relatedvalue, and the controller is located in a receiver that receives thedata from the transmitter.
 3. The apparatus as set forth in claim 1,wherein the controller judges whether or not the life of the battery isending based on a comparison between the voltage-related value and avariable voltage reference value that is selected in accordance with thetemperature in the tire.
 4. The apparatus as set forth in claim 3,wherein the controller determines the voltage reference value using afunctional equation that represents variation of the voltage of thebattery with respect to the ambient temperature to which the battery isexposed.
 5. The apparatus as set forth in claim 1, wherein thecontroller judges whether or not the life of the battery is ending basedon a comparison between the voltage-related value and a predetermined,constant voltage reference value if the temperature in the tire is equalto or greater than a predetermined reference value, and the controllerdoes not perform the judgment regarding the life of the battery if thetemperature in the tire is smaller than the reference value.
 6. Theapparatus as set forth in claim 3, wherein the controller determinesthat the life of the battery is ending if the voltage-related value hasbeen repeatedly judged to be smaller than the voltage reference value inconsecutive judgment cycles.
 7. The apparatus as set forth in claim 5,wherein the controller determines that the life of the battery is endingif the voltage-related value has been repeatedly judged to be smallerthan the voltage reference value in consecutive judgment cycles.
 8. Theapparatus as set forth in claim 1, wherein the voltage-related value isthe voltage of the battery or a resistance in an electric circuit of thetransmitter.
 9. A tire condition monitoring apparatus for monitoring acondition of a tire attached to a vehicle, the apparatus comprising: atransmitter, which is operated by a battery and wirelessly transmitsdata, wherein the data includes at least data that represents thetemperature in the tire and data that represents voltage of the battery;and a receiver, which receives the data from the transmitter, whereinthe receiver includes a controller that judges whether or not the lifeof the battery is ending depending on the voltage of the battery, whichis derived from the received data, wherein the judgment regarding thelife of the battery is performed in accordance with the temperature inthe tire, which is derived from the received data.
 10. The apparatus asset forth in claim 9, wherein the controller judges whether or not thelife of the battery is ending based on a comparison between the voltageof the battery and a variable voltage reference value that is selectedin accordance with the temperature in the tire.
 11. The apparatus as setforth in claim 10, wherein the controller determines the voltagereference value using a functional equation that represents variation ofthe voltage of the battery with respect to the ambient temperature towhich the battery is exposed.
 12. The apparatus as set forth in claim 9,wherein the controller judges whether or not the life of the battery isending based on a comparison between the voltage of the battery and apredetermined, constant voltage reference value if the temperature inthe tire is equal to or greater than a predetermined reference value,and the controller does not perform the judgment regarding the life ofthe battery if the temperature in the tire is smaller than the referencevalue.
 13. The apparatus as set forth in claim 10, wherein thecontroller determines that the life of the battery is ending if thevoltage of the battery has been repeatedly judged to be smaller than thevoltage reference value in consecutive judgment cycles.
 14. Theapparatus as set forth in claim 12, wherein the controller determinesthat the life of the battery is ending if the voltage of the battery hasbeen repeatedly judged to be smaller than the voltage reference value inconsecutive judgment cycles.
 15. A tire condition monitoring apparatusfor monitoring a condition of a tire attached to a vehicle, theapparatus comprising: a transmitter, which is operated by a battery andincludes: a temperature sensor, which detects the temperature in thetire; a voltage sensor, which detects the voltage of the battery; and atransmitting circuit, which wirelessly transmits data including datathat represents the detected temperature and data that represents thedetected voltage; and a receiver, which receives the data from thetransmitter, wherein the receiver includes a controller that judgeswhether or not the life of the battery is ending based on a comparisonbetween the voltage of the battery, which is derived from the receiveddata, and a variable voltage reference value that is selected inaccordance with the temperature in the tire, which is derived from thereceived data.